The present invention relates to an electrical control circuit and, more particularly, to an electrical circuit for controlling the application of power to a load that is initially activated and deactivated by one or more switches.
The use of solid state driver circuits to control the application of battery power to an automotive load device is becoming more and more prevalent. The load device is typically a lamp which may be integrated with an ON/OFF switch for primary control purposes. Such lamps may also be subject to secondary activation; for example, an automotive dome light may be turned ON and OFF both by the integrated ON/OFF switch and by a door ajar switch to light the interior of the automobile any time a door is opened.
An integrated switch cannot be wired as a separate input into an on-board microprocessor to provide a xe2x80x9cwake-upxe2x80x9d function. Therefore, it is required in such circuits to have the driver on even when no power to the load is required; i.e., when neither the integrated ON/OFF switch nor the door ajar switch is closed. Having the driver circuit, which typically controls a field effect transistor (FET), powered on at these times creates parasitic loss currents, an undesirable condition. The parasitic loss currents can exceed key-off load requirements for a vehicle.
The present invention addresses the problem described above by providing a driver circuit such as an FET which is normally OFF, and further providing a low power sense circuit connected to a controller to detect the closed switch condition and activate or xe2x80x9cwake upxe2x80x9d the driver circuit when full power is required. The invention can be applied to both high side and low side switching circuit designs and to loads, such as lamps, which are switched by both primary, integrated ON/OFF switches and remote switches such as door ajar switches. By monitoring switch position from the sense circuit, the controller activates the driver circuit only when a switch is closed. Thus, no parasitic current drain is produced when all control switches are open.
In the illustrated embodiments, the controller further includes a timer to determine how long a load has been activated. After a predetermined period of time has lapsed, the controller deactivates the driver circuit to shut off power to the load.
For example, if the switched load is a dome lamp, the controller can turn the lamp off after a predetermined period of time (e.g., ten minutes) even though the dome light switch or an associated door ajar switch remains closed. In this manner, the electrical control circuit can prevent vehicle battery drain when a load has been activated for a long period of time such as when a operator has left the vehicle with the dome light on or has failed to close a door.